A primary color or colour is a color that cannot be created by mixing other colors in the gamut of a given color space. Primary colors may themselves be mixed to produce most of the colors in a given color space: mixing two primary colors produces what is generally called a secondary color, mixing a secondary with a primary produces what is sometimes called a tertiary color. Traditionally, the colors red, yellow, and blue are considered to be primary pigments in the art world. However, this is not technically true, or is at least inaccurate. The three primary colors of pigment are magenta, yellow, and cyan. (Thus when calling "red, yellow, and blue" the primary colors of pigment, "red" is an inaccurate way of saying "magenta" and "blue" is an inaccurate way of saying "cyan"). Blue and green are actually secondary pigments, but they are primary colors of light, along with red.

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Biological basis

Primary colors are not a fundamental property of light but rather a biological concept, based on the physiological response of the human eye to light. Fundamentally, light is a continuous spectrum of wavelengths, meaning that there are actually an almost infinite number of colors. However, the human eye normally only contains three types of receptors called cones. These respond to specific wavelengths of red, green, and blue light. Humans and other species with three such types of color receptors are known as trichromats. Although the peak responsivities of the cones do not occur exactly at the red, green and blue frequencies, those three colors are chosen as primary because with them it is possible to almost independently stimulate the three color receptors, providing a wide gamut of experiences. Red Green and Blue are primary colors. To generate optimal color ranges for species other than humans, other additive primary colors would have to be used. For example, for species known as tetrachromats, with four different color receptors, one would use four primary colors (since humans can only see to 400 nanometers (violet), but tetrachromats can see into the ultraviolet to about 300 nanometers, this fourth primary color would be located in this range and would probably be a pure spectral magenta rather than the magenta we see which is a mixture of red and blue). Many birds and marsupials are tetrachromats and it has been suggested that some female humans are born as tetrachromats as well[1][2], having an extra receptor for yellow. On the other hand, most mammals have only two types of color receptors and are therefore dichromats; to them, there are only two primary colors.

Additive primaries

Media that combine emitted lights to create the sensation of a range of colors are using the additive color system.
Television is the most common use of this. The additive primaries are red, green,
and blue. Because of the response curves of the three different color receptors in
the human eye, these colors are optimal in the sense that the largest range
of colors — a gamut — visible by humans can be generated by mixing light of these colors.
Additive mixing of red and green light, produce shades of yellow or orange. Mixing green and blue produces shades of cyan, and mixing red and blue produces shades of purple and magenta. Mixing equal proportions of the additive primaries results in shades of grey; when all three colors are fully saturated, the result is white. The color space that is generated is called the RGB ("red, green, blue") color space.

Subtractive primaries

Media that use reflected light and colorants to produce colors are using the subtractive color method of color mixing. In the printing industry, to produce the varying colors, apply the subtractive primariesyellow, cyan, and magenta together in varying amounts. Subtractive color works best when the surface or paper is white, or close to it.

Mixing yellow and cyan produces shades of green; mixing yellow with magenta produces shades of red, and mixing magenta with cyan produces shades of blue. In theory, mixing equal amounts of all three pigments should produce shades of grey, resulting in black when all three are fully saturated, but in practice they tend to produce muddy brown colors. For this reason, a fourth "primary" pigment, black, is often used in addition to the cyan, magenta, and yellow colors.
The color space generated is the so-called CMYK color space. The abbreviation stands for "Cyan, Magenta, Yellow, and Black" — the black is referred to as K for key, a shorthand for the printing term "key plate" (the printing plate that impressed the artistic detail of an image, usually in black ink).

In practice, mixtures of actual materials like paint tend to be less precise. Brighter, or more specific colors can be created using natural pigments instead of mixing, and natural properties of pigments can interfere with the mixing. For example, mixing magenta and green in acrylic creates a dark cyan - something which would not happen if the mixing process were perfectly subtractive. In the subtractive model, adding white to a color does not change its hue but does reduce its saturation.